In many printing systems, it is common practice to develop a hardcopy of an image by using a photoconductive surface. The photoconductive surface is selectively charged with a latent electrostatic image having image and background areas. A liquid developer, i.e. an electrostatic ink, comprising charged toner particles in a carrier liquid is brought into contact with the selectively charged photoconductive surface. The charged toner particles adhere to the image areas of the latent image while the background areas remain clean.
Various techniques for electrostatic image transfer are known. One method includes the use of an intermediate transfer member. A liquid image, which includes a liquid carrier having ink particles dispersed therein, is transferred to a photoconductive member or drum and from there to a surface, e.g., a release layer or blanket, of the intermediate transfer member. The liquid image is electrostatically attracted from the photoconductive surface to the surface of the intermediate transfer member. The liquid carrier is removed from the surface of the intermediate transfer member and the ink particles are compacted on the surface in the image configuration. Thereafter, the ink particles are transferred from the surface of the intermediate transfer member to a substrate in the image configuration.
Modern liquid toner electrostatic imaging began with the invention of a new class of toners referred to as ElectroInk™. This type of toner is characterized by its toner particles being dispersed in a carrier liquid, where the toner particles include a core of a resin, i.e. a polymer, with fibrous extensions extending from the core. When the toner particles are dispersed in the carrier liquid in a low concentration, the particles remain separate. Although not intending to be bound by theory, when the toner develops an electrostatic image, the concentration of toner particles increases and the fibrous extensions interlock
Typically the toner comprises a thermoplastic resin (polymer) as the basis for the toner particles (also referred to herein as ink particles), and a non-polar liquid as a carrier liquid in which the toner particles are dispersed. Generally, the toner particles contain a colorant such as a pigment. An example of such a toner can for instance be found in U.S. Pat. No. 5,923,929.
A charge director, also called charge control agent or imaging agent, is also added to the dispersion to induce charge on the particles. Many of the prior art charge directors are mixtures of several charge director components having different chemistries. An example of such a charge director is disclosed in U.S. Pat. No. 5,346,796 in which the liquid ink further comprises a non-quaternary amine salt soluble in the carrier liquid such as isopropylamine dodecylbenzenesulfonate to stabilize the electrical properties of the charge director.
A possible disadvantage of using a mixture of materials as charge director is the possibility of selective adsorption of certain charge director components onto the surface of the ink particles. This may lead to differential depletion of the components responsive to their affinity to the ink particles. Therefore, a non-controllable change in the charge director composition may be expected during continuous printing process. This may adversely affect the long term characteristics of the charge director and reflected in degraded print quality.
Such problems have been overcome by certain types of micelle-based charge directors such as the charge director disclosed in WO2007/130069 A1, which provide improved print quality compared to charge directors having multiple charge directing components. Such charge directors are sometimes referred to as single charge directors (SCD) because of the fact that the ink composition only comprises a single complex compound as a charge director.
It has been found by the present inventors that some charge directors comprising only a single charge component can cause negative optical density memory effects originating from the intermediate transfer member during high frequency printing of several types of images including most notably (monochrome) images. Negative optical density memory is a phenomenon whereby the optical density of solids measured on the substrate decreases along the printing process. In extreme cases, this can cause the appearance of holes in the ink layer observed on the substrate.